build: Experimental support to build using precompiled headers

This type of build needs to be explicitly enabled using --with-pch
configure option.

Change-Id: I02dc304187d54c3cf40f1d51b1b11b21f868e225
diff --git a/tests/wscript b/tests/wscript
index 877d24a..4fbfb4a 100644
--- a/tests/wscript
+++ b/tests/wscript
@@ -19,3 +19,7 @@
 
     # In case we want to make it optional later
     unittests.source += bld.path.ant_glob('**/*-sqlite3.cpp')
+
+    if bld.env['WITH_PCH']:
+        unittests.pch = "../src/common.hpp"
+